feat: Consistent and sensible layout_config (#922)

* feat: Consistent and sensible layout_config

* [docgen] Update doc/telescope.txt
skip-checks: true

* [WIP]: Thu 17 Jun 2021 03:36:44 PM EDT

* [WIP]: Thu 17 Jun 2021 03:38:11 PM EDT

* layout_default -> layout_defaults

* remove options from bug repot

* Conni2461 suggestions: part 1

* [docgen] Update doc/telescope.txt
skip-checks: true

* Conni2461 suggestions: part 2

* [docgen] Update doc/telescope.txt
skip-checks: true

* Linting

* Improve deprecation checks

- Move `layout_defaults` handling to `deprecated.lua`
- Check for "layout keys" outside of `layout_config` on `setup`

* fixup: Just add a few more words

Co-authored-by: Luke Kershaw <35707277+l-kershaw@users.noreply.github.com>
Co-authored-by: Github Actions <actions@github>

lua/telescope/themes.lua

@@ -27,16 +27,26 @@ function themes.get_dropdown(opts)
   opts = opts or {}
 
   local theme_opts = {
-    -- WIP: Decide on keeping these names or not.
     theme = "dropdown",
 
+    results_title = false,
+    preview_title = "Preview",
+
     sorting_strategy = "ascending",
     layout_strategy = "center",
-    results_title = false,
-    preview_title = "Preview",
-    preview_cutoff = 1, -- Preview should always show (unless previewer = false)
-    width = 80,
-    results_height = 15,
+    layout_config = {
+      preview_cutoff = 1, -- Preview should always show (unless previewer = false)
+
+      width = function(_, max_columns, _)
+        return math.min(max_columns - 3, 80)
+      end,
+
+      height = function(_, _, max_lines)
+        return math.min(max_lines - 4, 15)
+      end,
+    },
+
+    border = true,
     borderchars = {
       { "─", "│", "─", "│", "╭", "╮", "╯", "╰"},
       prompt = {"─", "│", " ", "│", "╭", "╮", "│", "│"},